The present invention relates to a fuel cell using an enzyme as a catalyst, an electronic device, a movable body, a power generation system, and a cogeneration system.
A fuel cell basically comprises a fuel electrode (negative electrode), an oxidizer electrode or air electrode (positive electrode), and an electrolyte (proton conductor), and has an operational principle, in accordance with a reverse reaction of the electrolysis of water, such that hydrogen and oxygen are reacted to form water (H2O) and generate electricity. Specifically, a fuel (hydrogen) supplied to the fuel electrode is oxidized and divided into electrons and protons (H+), and the electrons go to the fuel electrode and the protons H+ travel through the electrolyte to the oxidizer electrode. At the oxidizer electrode, the protons H+ are reacted with oxygen supplied from the outside and electrons fed from the fuel electrode through an external circuit to form H2O.
The fuel cell is a high-efficient power generator which directly converts the chemical energy of a fuel to electrical energy, and can extract electrical energy from the chemical energy of fossil energy, such as natural gas, petroleum, or coal, with high conversion efficiency, irrespective of where or when the fuel cell is used. For this reason, conventionally, fuel cells in large-scale power generation applications and others have been extensively researched and developed. For example, fuel cells are mounted on a space shuttle, which has demonstrated that the fuel cells can supply not only electric power but also water for a crew, and that the fuel cells are clean power generators.
Further, in recent years, fuel cells operating in a range of relatively low temperatures of from room temperature to about 90° C., such as solid polymer fuel cells, are developed and have attracted attention. Therefore, attempts are being made to apply the fuel cell not only to the large-scale power generation but also to small-size systems, such as a power source for driving an automobile, and a portable power source for personal computer or mobile device.
As mentioned above, the fuel cell is possibly applied to a wide range of uses from the large-scale power generation to the small-scale power generation, and has attracted considerable attention as a high-efficient power generator. However, the fuel cell has various problems in that the fuel cell generally uses, as a fuel, hydrogen gas converted by means of a reformer from natural gas, petroleum, or coal, and hence consumes limited resources and requires high-temperature heating, and that the fuel cell needs a catalyst comprised of an expensive noble metal, such as platinum (Pt). In addition, when hydrogen gas or methanol itself is directly used as a fuel, it must be carefully handled.
For solving the problems, the application of biological metabolism proceeding in a living body, which is a high-efficient energy conversion mechanism, to a fuel cell has been proposed. The biological metabolism used here includes respiration, photosynthesis, and the like conducted in microorganism somatic cells. The biological metabolism has advantageous features not only in that the power generation efficiency is extremely high, but also in that the reaction proceeds under mild conditions at about room temperature.
For example, respiration is a mechanism such that microorganisms or cells take in nutrients, such as saccharides, fat, and protein, and, during the formation of carbon dioxide (CO2) through the glycolytic pathway and tricarboxylic acid (TCA) cycle having a number of enzyme reaction steps, nicotinamide adenine dinucleotide (NAD+) is reduced to form reduced nicotinamide adenine dinucleotide (NADH), thus converting the chemical energy of the nutrients to redox energy, i.e., electrical energy, and further, in the electron transport system, the electrical energy of NADH is directly converted to proton-gradient electrical energy and oxygen is reduced to form water. The resultant electrical energy forms ATP from adenosine diphosphate (ADP) through adenosine triphosphate (ATP) synthase, and ATP is utilized in the reactions required for living of microorganisms or cells. This energy conversion is carried out in cytosol and mitochondria.
Photosynthesis is a mechanism such that, during the conversion of the optical energy taken in to electrical energy by reducing nicotinamide adenine dinucleotide phosphate (NADP+) through the electron transport system to reduced nicotinamide adenine dinucleotide phosphate (NADPH), water is oxidized to form oxygen. The resultant electrical energy is utilized in a carbon fixing reaction by taking in CO2 and a synthesis of carbohydrates.
As a technique for utilizing the above-mention biological metabolism in a fuel cell, a microorganism battery in which electrical energy generated in microorganisms is removed from the microorganisms through an electron mediator and the resultant electrons are delivered to an electrode to obtain an electric current has been reported (see, for example, Unexamined Japanese Patent Application Laid-Open Specification No. 2000-133297).
However, microorganisms and cells have many functions other than the desired reactions including the conversion of chemical energy to electrical energy. Therefore, in the above method, an undesired reaction consumes the electrical energy, making it difficult to achieve a satisfactory energy conversion efficiency.
For solving the problem, a fuel cell in which only a desired reaction is advanced using an enzyme and an electron mediator has been proposed (see, for example, Japanese Patent Application Publication Nos. 2003-282124 and 2004-71559). In this fuel cell, a fuel is decomposed into protons and electrons by an enzyme, and fuel cells using as a fuel an alcohol, such as methanol or ethanol, or a monosaccharide, such as glucose, have been developed.
However, the above-mentioned conventional fuel cell using alcohol or glucose as a fuel is unsatisfactory in power generation efficiency, and hence is difficult to put into practical use.
Accordingly, a task to be achieved by the present invention is to provide a fuel cell which is advantageous not only in that it can directly extract electric power from a polysaccharide to achieve high-efficient power generation, but also in that it does not require limited fossil fuel and contributes to the realization of resource circulation society.
Another task to be achieved by the present invention is to provide an electronic device, a movable body, a power generation system, and a cogeneration system using the above excellent fuel cell.